1. Field of the Invention
The invention relates to height control valves in vehicle suspensions and, more particularly, to a height control valve with a novel dump valve. In another of its aspects, the invention relates to a dump valve for a height control valve.
2. State of the Prior Art
Air suspension systems have become increasingly popular for use in vehicle suspensions, seats, and cabs on semi-trailer truck rigs and other vehicles. Most typically employ a height control valve which maintains the design height of the suspension. In a semi-trailer suspension, for example, the design height is the spacing between the frame and the axle. The height control valve senses when the spacing is greater or less than its design value and, accordingly, adjusts the pressure in the air springs disposed between the frame and axle. The air suspension system with a height control valve maintains a uniform height of the air spring over a range of vehicle loading conditions.
The height control valve operates by selectively admitting air into, or exhausting air from, an air spring. The air spring is positioned between the truck frame and a trailing arm. The trailing arm carries the axle so that adjusting the air spring pressure also adjusts the distance between the truck frame and the axle. The height control valve typically mounts on the truck frame and has a control arm which connects to the trailing arm through a linkage. As the distance between the truck frame and trailing arm varies, the linkage causes the control arm to rotate a control shaft inside of the height control valve, which, in turn, selectively permits air to be admitted to or exhausted from the air spring.
Typically, the height control valve has three ports: an air spring port connected to the air spring, an inlet port connected to a source of pressurized air, and an exhaust port open to the atmosphere. To increase the height of the air spring, the height control valve opens fluid communication between the air spring port and the inlet port, allowing air to flow from the pressurized air source, through the height control valve and into the air spring. To decrease the height of the air spring, the height control valve opens fluid communication between the air spring port and the exhaust port, allowing air to flow out of the air spring through the height control valve, and to atmosphere through the exhaust port. When the air spring is at the proper height, the valve is in a neutral position wherein neither the inlet nor exhaust port communicates with the air spring.
Typically, dump valves are used on tractors and trailers equipped with air suspensions to exhaust air from the air springs and lower the vehicle chassis height. Common applications for dump valves on tractors would be to exhaust air from the tractor suspension before the tractor is connected or disconnected from a trailer. The operator typically lowers the front dolly legs on the trailer until they touch the ground. The gear mechanism on the dolly legs is manually operated and is thus easier to turn with no load on the legs. The air is then "dumped" or exhausted from the tractor air springs, lowering the tractor frame and thereby shifting the trailer weight to the dolly legs. The tractor is then free to separate from the trailer.
Dump valves are often used on trailers to lower the vehicle frame height during loading conditions. A condition known as trailer creep sometimes occurs as a trailer is loaded adjacent a loading dock. If the height of the trailer is not controlled during loading, the increasing load in the trailer will sometimes cause the trailer tires to rotate about the ground contact point and force the trailer forward. In other words, the trailer tends to "walk" away from the loading dock. This tendency presents obvious problems with strain levels on the trailer dolly legs and access to the trailer from the dock.
Typically, prior art dump valves used to exhaust air from the air springs are separate mechanisms which must be positioned and mounted on the vehicle and separately plumbed into the air lines. The additional labor and hardware costs often exceed the cost of the dump valve itself and increase the time for vehicle assembly.